Chromate conversion coatings are widely used in the manufacture, repair, and refurbishment of a large number of engineering components for military and civilian applications. These surface coatings resist corrosion and promote paint adhesion. Such protective coatings are applied to a variety of substrate metals and alloys, most notably aluminum. Conversion coatings are produced by either immersion (dipping), spraying, swabbing, or brushing techniques that contain chromates and dichromates in the processing solution. Occasionally, electrolytic methods are used to obtain conversion coatings, but such methods require a great deal of maintenance and are impractical for recoating parts in the field.
Chemical conversion coatings are formed by a chemical reaction causing the surface of the metal to be converted into a tight adherent coating, all or part of which consists of an oxidized form of the substrate metal. The coating can provide high corrosion resistance as well as strong anchoring for paint. The industrial application of paint (organic finishes) to metals generally requires the use of a chemical conversion coating as a base coating, particularly when the performance demands are high.
Although aluminum protects itself against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and electrolytes, aluminum alloys, particularly the high-copper aluminum alloys, corrode much more rapidly than pure aluminum. Thus, there is a need to treat aluminum with some form of beneficial conversion coating.
Generally two types of conversion coating processes are used in treating aluminum. The first is by anodic oxidation (anodization) in which the aluminum component is immersed in a chemical bath, such as a chromic or sulfuric acid bath, and an electric current is passed though the aluminum component and the chemical bath. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes.
The second type of process chemically produces a conversion coating by subjecting the aluminum surface to a chemical solution, but without the use of electric current. This process is commonly referred to as a chemical conversion coating. The chemical solution can be applied by using immersion or spray application and is followed by drying. When dried, the coating which is initially gelatinous (gel) hardens, and becomes hydrophobic (less soluble in water) and more resistant to abrasion. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes, such as a paint top coat.
Commonly, a chromate conversion coating solution containing chromium ions in the hexavalent [Cr(VI)] and trivalent [Cr(III)] state is used to produce a chemical conversion coating. The Cr(VI) is partially reduced to Cr(III) during the reaction, with a concurrent rise in pH. The chemical composition of the surface coating is indefinite as it contains varying amounts of reactants, reaction products, water of hydration and other anions, such as fluorides, and phosphates. When hexavalent Cr (VI) ions are incorporated into a coating, the ions leach out when in contact with a moisture and thereby provide corrosion resistance and also impart paint adhesion properties to the coating. However, solutions containing chromium ions in the hexavalent state have been determined to be carcinogenic. The U.S. Environmental Protection Agency (EPA) has included chromium on the list of toxic chemicals for "voluntary" replacement, and has promulgated strict waste disposal standards to curtail its use.
Strict waste disposal standards and chromium's listing as a toxic chemical have created a need for alternative chemical conversion coating compounds that do not contain the Cr(VI) ion. For such a compound to be accepted as an alternative it must meet or exceed the protective properties displayed by the chromium compounds. An alternative must also be capable of being used as a substitute with very little modifications to the present process so that it is readily accepted. An alternative providing protection comparable to that of chromium without being toxic is needed.